The present invention relates to a process for production of aromatic hydroxycarboxylic acids.
Aromatic hydroxycarboxylic acids are important for raw materials or intermediates for antiseptics, antifungals, pigments, dyes, liquid crystals, pharmaceuticals, agricultures and the like, which are generally produced by the reaction of alkaline metal salts of phenolic or naphtholic compounds with carbon dioxide.
As such a method a solid-vapor phase reaction called as Kolbe-Schmitt reaction has been used, but it has many problems attributed to the solid-vapor phase reaction, for instance, a long reaction time, a great loss of raw materials by side-reaction which is caused by a thermal unevenness, an inconstant yield due to difficulty in reaction control and so on. Various improvements proposed to solve the above problems have not satisfied industrial requirements.
Japanese Patent Publication No. 9529/1970 (corresponding to U.S. Pat. No. 3,816,521) discloses a method of continuous production of p-hydroxybenzoic acid by the reaction of phenol and carbon dioxide, in which as a phenolic compound potassium salt of phenol and free phenol are used in a given ratio dispersed in a specific reaction medium such as aliphatic hydrocarbons, aromatic hydrocarbons and the like, but it does not refer to the removal of byproducts from the reaction system as a vapor. Japanese Patent Publication No. 53296/1981 (corresponding to U.S. Pat. No. 4,239,913) discloses the production of 2-hydroxynaphthalene-3-carboxylic acid by the reaction of sodium salt of .beta.-naphthol with carbon dioxide, in which the reaction is carried out in the dispersion of mixture of the sodium salt and .beta.-naphthol in a specific reaction medium such as a light oil in a given ratio. This method does not refer to the removal of by-products produced during the reaction as a vapor from the reaction system, either. The both methods are based on the fact that the mixture of a free phenolic or naphtholic compound and its alkaline metal salt in a specific ratio can be maintained liquid at a reaction temperature, and the contact of the salts with the carbon dioxide is advantageously achieved in the dispersion of the alkaline metal salts in a specific reaction medium. According to these methods there are improved in productivity, purity of aromatic hydroxycarboxylic acid produced, yield as well as the reaction can be continuously operated and completed within short time.
As apparent from the above the mixture of the phenol (naphthol) and its alkaline metal salts in the reaction medium (such a mixture is referred to as a reaction mixture through the present specification, and the alkaline metal salts alone in the reaction medium is also referred to a reaction mixture) should be maintained in a specified component ratio over the reaction process, but the amount of free phenols or free naphthols discharged from the reaction system is practically depended on the amount of the reaction medium. Therefore, these both processes still have following difficulties:
(1) If the reaction mixture is maintained for a long time in the reaction system, free phenols or free naphthols produced during the reaction accumulate more than the specified ratio in the reaction system so as to be beyond the suitable ratio, and then by-products such as tars from phenols, naphthols and the like are still liable to increase; and
(2) If excess amount of phenols or naphthols is discharged to decrease the by-products, the amount of the reaction medium relatively increases, and the residence time of the reaction mixture becomes shorter, so that the conversion ratio from alkaline metal salts of phenols (naphthols) to the corresponding hydroxycarboxylic acids decreases.
Additionally, as, when a batch reaction is used, it has naturally no means to discharge the phenols or naphthols produced during the reaction in these methods, the specified ratio of the reaction mixture cannot be suitably maintained. Therefore, higher yield cannot be expected.